Piston for an internal combustion engine

ABSTRACT

A piston for an internal combustion engine of the type presenting circumferential grooves ( 10 ), each groove ( 10 ) housing a respective piston ring ( 20 ), at least one first upper groove ( 10 ) presenting a profile having upper and lower lateral walls ( 11, 12 ) radially outwardly inclined towards the piston top by an angle of inclination such as to compensate, at least partially, the deformations to which the piston is submitted when in a critical higher load operational condition, in order to maximize the distribution of the seating contact between at least one of the upper and lower lateral walls ( 21, 22 ) of the piston ring ( 20 ) and an adjacent lateral wall ( 11, 12 ) of the groove ( 10 ), as well as to minimize the wear that determines the useful life of the groove ( 10 ).

FIELD OF THE INVENTION

The present invention refers to a constructive solution for a piston ofthe type used in an internal combustion engine and, more particularly,to a constructive solution for the groove of such piston.

BACKGROUND OF THE INVENTION

The piston ring of internal combustion engines presents, due to assemblyor operational clearances, a relative movement in relation to thegroove. Such relative movement, associated to the load imparted to thering, mainly by the combustion gases, causes wear to the lateral facesof both the ring and the groove. In the groove located closer to thepiston top, or first groove, where the loads are more severe, the ringis made of cast iron or steel, while the piston is of aluminum alloy,especially for Otto cycle engines. As the material of the piston is lesswear resistant than that of the ring, more concern about wear isconcentrated on the piston groove.

Under normal conditions, the wear of the first groove is of the order offew micrometers throughout the useful life of the engine, not impairingthe engine's performance. In engines having severe operationalconditions, in which the wear would be excessive, it is commonly usedthe solution of hard anodizing the region of the first groove, whichsolution creates a wear resistant hard flank, leading to acceptable wearvalues. However, such solution has the disadvantage of increasing thepiston cost in about 20%.

In recent years, the increase in the engines' specific power has beenassociated to the use of rings made of nitrided steel, which, althoughbringing advantages as to the consumption of lubricant oil and thesealing of combustion gases, on the other hand can increase the groovewear, since the rings are harder than the cast iron. Thus, it has beenmore common to occur problems of excessive wear, consequently impairingthe engine's performance, at least during the development phase of theengine. This wear problem has been solved by using nobler aluminumalloys and/or by hard anodizing the piston. However, both solutionsincrease the cost of the product or lead to the use of cast iron rings,avoiding the use of the significant advantages of employing rings ofnitrided steel.

As mentioned above, it is possible to increase the wear resistance ofthe groove by using, in the piston, nobler and more wear resistantaluminum alloys, or by hard anodizing. Since the wear resistance of thealuminum is considerably reduced with temperature increase, someartifices to reduce the temperature may be used, especially in theregion of the first groove. It is known to use spraying a lubricantthrough injecting nozzles located in the engine block in the internalregion of the piston, in which case the lubricant oil functions as arefrigerant. It is also possible to locate the first groove more distantfrom the piston top, which reduces its temperature, but bringsdisadvantages as to the emission of pollutants by the engine.

Ideally, the lateral faces of both the ring and the groove of the pistonshould be parallel, so that the ..contact and the resulting pressurescan be distributed, which minimizes the wear (FIG. 1). However, due todesign characteristics, thermo-mechanical deformations of the piston, orto the relative angular movement between the ring and the piston, suchcontact occurs, in determined operational conditions of the piston, in acontact region between the ring and the interior of the groove (FIG.1A).

Due to the differentiated thermal expansion of the piston, higher at thetop where the temperatures are higher, and lower towards its lowerportion, which is commonly denominated piston skirt, the first groovetends to change its nominal design inclination, in a cold condition, toa higher inclination downwardly (FIG. 1A). Typical values of thisinclination change are of the order of 10-15 minutes, in theanticlockwise direction, i.e., the groove, under operation, tends tochange its nominal inclination to a higher inclination downwardly.

It is known to use pistons with grooves that are upwardly inclined intheir nominal values. This is usually made to assure the ring will notcontact the cylinder wall with its upper portion, which would beundesirable as to the scraping of lubricant oil by the ring. In additionto the thermal deformation of the groove, the piston moves angularly inrelation to the pin, so that the resulting angle depends on the positionof the pin along the height of the piston. This movement is shown inFIG. 2, in which the maximum displacement of the piston for each side ofa plane that is orthogonal to the piston axis is illustrated. Themaximum inclination of the piston as a whole is of about 10 minutes, andit can vary at each instant of the piston stroke. The effect of suchinclination in groove wear is quite inferior to that resulting fromgroove inclination, which lasts throughout the piston stroke.

Due to the transient conditions found in the internal combustion engine,in which at each l degree interval of the crankshaft (which, forexample, at 3,000 rpm is equivalent to about 0.06 millisecond), thering/groove relative position, as well as the ring load on the groove,vary during the combustion stroke, as well as in each operationalcondition of the engine.

In a known prior art solution (JPI-182679), the lateral face of the ringis provided with the same angle of inclination as the groove underoperation (FIG. 1C). In this construction, the ring has its lateral facewith the same inclination as that of the groove under operation.

Rings having the lower lateral face inclined, as proposed in thedocument above, with either a trapezoidal or a semi-trapezoidalcross-section, are used in diesel engines to avoid sticking of the ringby the carbon deposited in the groove and present the disadvantage ofhaving a much higher manufacturing cost than the rings with arectangular section.

OBJECT OF THE INVENTION

The object of the present invention is to provide a piston for aninternal combustion engine, which allows the contact between the pistonring and the groove, during operation, especially in the moments ofhigher pressure on the ring, to be as distributed as possible, in orderto minimize the wear rate of the lateral walls of said groove.

SUMMARY OF THE INVENTION

This and other objects are achieved by a piston for an internalcombustion engine of the type presenting circumferential grooves, eachgroove housing a respective piston ring and at least one first uppergroove having a profile with upper and lower lateral walls that areradially outwardly inclined towards the piston top, by an angle ofinclination such as to compensate, at least partially, the deformationsto which the piston is submitted when in a critical higher loadoperational condition, in order to maximize the distribution of theseating contact between at least one of the upper and lower lateralfaces of the ring and an adjacent lateral wall of the groove, as well asto minimize the wear that determines the useful life of the groove.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below, with reference to the appendeddrawings, in which:

FIGS. 1, 1 a and 1 b show, respectively and schematically, longitudinalvertical sectional views of prior art constructions of a piston for aninternal combustion engine, mounted inside a cylinder and carrying, in afirst groove, a respective piston ring, according to the prior art;

FIG. 2 is a vertical lateral view of a piston for an internal combustionengine, illustrating the directions of the angular displacement of saidpiston in relation to a plane orthogonal to the longitudinal axis ofsaid piston;

FIG. 3 illustrates, schematically, the worn profile of the first grooveof the piston, said groove being made according to the prior artillustrated in FIG. 1;

FIG. 4 illustrates, schematically, the profile of a groove constructedin accordance with the present invention; and

FIG. 4 a illustrates, schematically, the worn profile of the pistongroove constructed according to the present invention and illustrated inFIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in relation, for example, apiston designed to reciprocate inside a cylinder C of an internalcombustion engine, and which is of the type illustrated in FIG. 2,usually made of aluminum or aluminum alloys and having a plurality ofcircumferential grooves 10, each groove 10 housing a respective pistonring 20.

The piston ring 20 is formed of a harder material than that of thepiston, for example, steel, cast iron or a sintered metallic alloy, andgenerally presents an annular body having an upper lateral face 21 and alower lateral face 22, which are opposite and generally parallel to eachother and orthogonal to the axial axis of the ring, an internal face 23,and an external contact face 24 to be seated against an internal face ofthe cylinder C.

According to the present invention, at least the first groove 10presents a profile with an upper lateral wall 11 and a lower lateralwall 12, which are radially outwardly inclined towards the piston top,by a nominal angle of inclination such as to compensate, at leastpartially, the deformations to which the piston is submitted when in acritical higher load operational condition, in order to maximize thedistribution of the seating contact between at least one of the upperand lower lateral faces 21, 22 of the piston ring 20 and an adjacentlateral wall 11, 12 of the groove 10, as well as to minimize the wearthat determines the useful life of the groove 10, particularly on thelower lateral wall 11 of said groove 10.

According to the present invention, the maximization of the contactdistribution is achieved between the lower lateral wall 12 of the groove10 and the adjacent lower lateral face 22 of the piston ring 20, saidmaximization condition occurring when the lower lateral wall 12 of thegroove 10 is situated substantially coplanar with a plane orthogonal tothe longitudinal axis of the piston and an adjacent lower lateral face22 of the piston ring 20 is substantially seated on said lower lateralwall 12 of the groove 10, in the operational condition that determinesthe useful life of the groove 10.

In the illustrated construction, the first groove 10 has the respectiveupper and lower lateral walls 11, 12 parallel to each other and thepiston ring 20 has its upper and lower lateral faces 21, 22 parallel toeach other.

While the drawings illustrate only one constructive form for the upperand lower lateral faces 21, 22, it should be understood that they mayhave other configurations, such as defining, for example, a trapezoidalor semi-trapezoidal profile for said piston ring 20. It should befurther understood that the upper and lower lateral walls 11, 12 of eachgroove 10 may equally have configurations other than being parallel toeach other, as illustrated.

According to the present invention, the angle of inclination of thegroove 10 is defined as a function of the mechanical and thermalcharacteristics of the piston and of the material with which it isformed, said thermal characteristics being determined by thecoefficients of thermal transmission and thermal expansion of the pistonmaterial, and the mechanical characteristics of the piston ring beingdetermined by the torsion and rigidity stiffness of the respectivecross-section of the piston ring 20.

The achievement of the angle of inclination in accordance with thepresent invention also takes into account: the dynamics of the pistonand piston ring 20 together, foreseeing the pressures that said pistonring 20 will exert against the lower lateral wall 12 of the groove 10 ateach instant; the relative movement between each piston ring 20 and therespective groove 10; the wear rate of a portion of said lower lateralwall 12 of said groove 10; and the superficial roughness in one of theparts defined by the piston ring and the respective groove 10. The wearbetween two pieces with relative movement is determined by the relation(Archard's law):Q=(K.W/Hv)ΔSwhere: Q: volume of the material removed by wear

-   -   K: wear coefficient of the system    -   W: applied normal load    -   Hv: hardness, in Vickers, of the softer material    -   ΔS: sliding distance

Thus, it is possible to define the wear rate at each time intervalduring the combustion stroke, ΔWL (Wear Load), as:ΔWL=Q/ΔS=(K.W)/Hvand the wear during the engine cycle as the summing up of the ΔWLsthroughout the stroke. In a lubricated regime, like that of thering/piston, part of the load W is supported by the hydrodynamicpressures of the lubricant film and these do not produce a significantwear.

The wear of the first groove 10 can be reduced by a groove/ringintegrated design that minimizes the wear rate in the criticaloperational condition. Particularly, this design takes into account: theinclination change of the groove 10, due to the operationaltemperatures; the secondary movement of the piston around its pin; andthe movement of the piston ring 20 in relation to the respective groove10.

Since the piston profile when heated inclines downwardly in relation tothe design position, the first groove 10, if this inclination change isnot properly compensated, will have the contact of the respective pistonring 20 with the lower lateral wall 11 of the groove 10 occurring in alocalized point, close to the inner bottom portion of said groove 10,starting an excessive wear process. In the initial stage, small cratersappear near said inner bottom portion of the groove 10.

FIG. 3 illustrates a prior art groove 10 in which its bottom portion hasbeen worn by the piston operating during a time interval of 150 hours,and in which the material resulting from this wear has been removedafter said time interval has elapsed. The engine operation causes wearin the groove 10 that is propagated towards the edge of the latter,producing a step that can reach about 0.30 mm (FIG. 3), with prejudicialconsequences to the engine's performance and even breaking the pistonring 20 or the piston itself.

In accordance with the present invention and as illustrated in FIGS. 4and 4 a, at least one groove 10 of the piston should present an angle ofinclination turned upwardly, towards the piston top of about, forexample, 5-30 minutes and preferably between 5 and 15 minutes, in orderto compensate for the downward inclination that the groove suffers underoperation. The specific value of this inclination depends on theproperties of the piston material, such as thermal conductibility andcoefficient of thermal expansion, on the critical or more significantoperational condition regarding wear rate, and on the dynamics of thepiston ring 20.

The upward inclination of the groove 10 allows that, under operation inthe selected operational condition, the dynamics and the lateral contactof an end lower face 22 of the piston ring 20 with the lower lateralwall 12 of the groove 10 results in a minimum wear rate.

The present invention has been tested in 3 gasoline engines and theresult is presented in Table 1, in which is shown the maximum wear valuefound in the lower lateral wall 11 of the first groove 10, before andafter design modifications. The engine identified as I began to presentexcessive wear of the groove 10 in the development phase period, whenits power has been increased. Engines II and III use hard anodizedpistons. The wear values shown in the original design refer to thevalues obtained with non-anodized pistons and maintaining the originaldesign. TABLE I I- Maximum wear found in the lower flank of the 1st.groove (μm) Engine Original design Optimized design I 1.0 L, 48 kW at 10 μm after 5 μm after 5,800 rpm 150 hrs 150 hrs II 1.6 L, 70 kW at  30μm after 2 μm after 5,500 rpm 150 hrs 150 hrs III 1.0 L, 44 kW at 300 μmafter 6 μm after 6,000 rpm 150 hrs 150 hrs

As it can be noted in FIG. 4 a, the profile of the groove 10 measured inthe maximum wear position shows that the optimized design not onlydrastically reduced the wear of the groove 10, allowing the use ofconventional aluminum piston alloys, but also demonstrate that the wearmechanism has been effectively altered. In the original design, thering/groove contact was concentrated near the inner portion of thegroove 10 whereas, after optimization, the worn profile of the groove 10has less wear and localized adjacent to the open edge of said groove 10,defining a trumpet like shape to the latter.

The present invention allows the contact of the ring with the lowerflank of the groove 10 under operation, especially in the moments ofhigher pressure on the piston ring 20, to occur as distributed aspossible, in order to minimize the wear rate in the lower flank of thegroove 10.

1. A piston for an internal combustion engine of the type presentingcircumferential grooves (10), each groove (10) housing a respectivepiston ring (20), characterized in that at least one first upper groove(10) presents a profile having an upper lateral wall (11) and a lowerlateral wall (12), which are radially outwardly inclined towards thepiston top by an angle of inclination such as to compensate, at leastpartially, the deformations to which the piston is submitted when in acritical higher load operational condition, in order to maximize thedistribution of the seating contact between at least one of the upperand lower lateral faces (21, 22) of the piston ring (20) and an adjacentlateral wall (11, 12) of the groove (10), as well as to minimize thewear that determines the useful life of the groove (10).
 2. The pistonas set forth in claim 1, characterized in that the maximization of thecontact distribution is achieved between a lower lateral wall (12) ofthe groove (10) and an adjacent lower lateral face (22) of the pistonring (20).
 3. The piston as set forth in claim 2, characterized in thatthe condition of maximization occurs when the lower lateral wall (12) ofthe groove (10) is situated on a plane that is substantially orthogonalto the longitudinal axis of the piston under operation conditions. 4.The piston as set forth in claim 2, characterized in that the firstgroove (10) has its upper and lower lateral walls (11, 12) parallel toeach other.
 5. The piston as set forth in claim 2, characterized in thatthe piston ring (20) has its upper and lower lateral faces (21, 22)parallel to each other.
 6. The piston as set forth in claim 1,characterized in that the angle of inclination for each groove (10) isdefined as a function of the mechanical and thermal characteristics ofthe piston ring (20), of the piston, and of the material that forms thelatter.
 7. The piston as set forth in claim 6, characterized in that thethermal characteristics of the piston are determined by the coefficientsof thermal transmission and thermal expansion of the piston material. 8.The piston as set forth in claim 7, characterized in that the mechanicalcharacteristics of the piston ring (20) are determined by the torsionand rigidity stiffness of the respective cross-section of the pistonring (20).
 9. The piston as set forth in claim 8, characterized in thatthe mechanical characteristics are determined taking into account therelative movements between the piston ring (20) and the respectivegroove (10), and the superficial roughness in each of the parts definedby the piston ring (20) and groove (10).
 10. The piston as set forth inclaim 1, characterized in that the angle of inclination is defined fromabout 5-30 minutes.
 11. The piston as set forth in claim 10,characterized in that the angle of inclination is preferably definedfrom about 5-15 minutes.